This grant is focused on dissecting at the molecular level the process by which retroelements like the yeast Ty1 element and the oncogenic retroviruses integrate into host genomes. The process of integration is known to be mediated by the integrase (IN) enzyme for retroviruses and retrotransposons like Ty1, which have long terminal repeats (LTRs). However, the roles of host proteins in integration are not well understood. Moreover, there are clear indications that the specificity of integration (that is, the selection of target sites in the host genome) is a non-random process, especially in vivo. This indicates the involvement of host factors in specifying integration sites, but the mechanism by which this occurs is not understood. Therefore, dissection of this problem in a genetic system like the budding yeast Saccharomyces cerevisiae is especially valuable. In addition, for the Ty1 system we have worked out in vitro systems for Ty1 integration that are quite powerful. Systems for studying integration in two other genetically manipulable systems, fission yeast (Schizosaccharomyces pombe) and the fruit fly are also available. In addition, we are studying the integration events mediated by the human L1 retroelement, which lacks LTRs and integrates using an unknown mechanism that is likely to be fundamentally different from the LTR element mechanism. Retroelement integration events often lead to genetic alterations such as activation of genes such as oncogenes or inactivation of tumor suppressor genes, which are critical steps in the development of a variety of animal and human cancers. Finally, we have developed powerful new yeast genetic systems for the analysis of macromolecular interactions, based on the two-hybrid system of Fields and coworkers and related systems. In particular, we developed a selective system in which molecules or macromolecules that dissociate or reverse a protein-protein or protein-DNA interaction can be identified. We will use these systems to analyze oncologically relevant macromolecular interactions as well as interactions between integrase and host factors.